Present day bridge structures reflect technological advances in design and construction that have evolved over the years. However, these advances have not precluded unfortunate and, in some instances, tragic occurrences of bridge collapses resulting in the loss of life and property. Older bridges are even more prone to failure. These events have increased interest in the inspection and maintenance of bridges, prompting the U.S. Congress in 1968 to require the establishment of national bridge inspection standards (NBIS) and the development of a program to train bridge inspectors. In subsequent years, the government has continued to stress the importance of bridge inspection and has ordered steps to insure that each state has a well-founded underwater inspection program including scour investigations. Recent revisions to the NBIS now mandate that a master list be developed of all bridges which require underwater inspection, that procedures be determined for these underwater inspections, and that the frequency of inspection for each bridge (not to exceed 5 years) be determined.
Surveys indicate that there are hundreds of thousands of bridges in the continental United States which are over waterways of varying widths and depths. Nearly one hundred thousand of these bridges must be monitored for scour conditions once every two years or more often if water conditions, such as floods, dictate such inspection. A scour condition can arise when abnormally high or unusually fast flowing water in a stream or river bed causes soil to be temporarily or permanently removed from various places in the river bottom, thus creating bottom holes or depressions which may have substantial depth in relation to the average elevation of the river bottom at these places. Although loose silt and other water- born materials may sometimes fill in a scoured area after the flood waters recede, this fill material often is less dense or compacted than the previously undisturbed bottom soil and thus does not offer as much lateral resistance as did the original soil. If such scouring occurs around a supporting pier or footing of a bridge so as to significantly diminish the lateral restraining forces applied to the structure by the surrounding soil, the potential exists for a failure of the structure after several scouring events have occurred. Thus, scour inspection is extremely important for bridge safety.
The two most common methods of scour inspection are manned diving operations and small craft fathometer surveys. However, these methods are usually practical for safety reasons only when the water is flowing less than about 3 feet per second. On the other hand, the most severe scour conditions occur during flood stages where water can flow in excess of 15 feet per second, which is when scour inspection is most useful in order to provide accurate data about the true extent of soil removal. The reason for inspecting during flooding is that, as mentioned above, loose silt may later refill a scoured area around a bridge footing after the water subsides, which then may give a false impression of soil conditions to a diver or to mechanical means for measuring the elevation of the river bed around bridge footings during a calm period. The present invention, however, is designed to function in fast flowing flood waters without posing danger to the personnel operating this system.